1. Field of the Invention
The invention relates to a collector for a projection exposure apparatus which is operated in a scanning mode along a scanning direction with a wavelength ≦193 nm, preferably ≦126 nm, more preferably with wavelengths in the extreme UV (EUV)-region. Said collector receives light from a light source and illuminates an area in a plane to be illuminated. The collector comprises a plurality of rotational symmetric mirror shells which are arranged within each other about a common rotational axis.
The invention further also provides an illumination system with such a collector, a projection exposure apparatus with an illumination system in accordance with the invention as well as a method for the exposure of microstructures.
2. Description of the Related Art
Nested collectors for wavelengths ≦193 nm, especially wavelengths in the region of X-rays, have become known from a plurality of documents.
U.S. Pat. No. 5,768,339 shows a collimator for X-rays, with the collimator comprising several paraboloidal reflectors. The collimator according to U.S. Pat. No. 5,768,339 is used to form into a parallel beam a isotropically radiated ray beam of an X-ray source.
A nested collector for X-rays has become known from U.S. Pat. No. 1,865,441 which is used—as in the case of U.S. Pat. No. 5,768,339—to collimate isotropic X-rays into a parallel beam of rays.
U.S. Pat. No. 5,763,930 shows a nested collector for a pinch plasma light source which is used to collect the radiation emitted by a light source and to focus the same in a waveguide.
U.S. Pat. No. 5,745,547 shows several arrangements of multi-channel optics which are used for concentrating the radiation of a source, especially X-rays, in a point. In order to achieve a high transmission efficiency, the invention according to U.S. Pat. No. 5,745,547 proposes elliptically shaped reflectors.
An arrangement has been disclosed in DE 30 01 059 C2 for the use in X-ray lithography systems which comprises parabolic mirrors arranged in a nested way between X-ray source and mask. These mirrors are arranged in such a way that the diverging X-rays are shaped into a parallel beam leaving the arrangement.
The arrangement according to DE 30 01 059 is merely used for achieving a favorable collimation for X-ray lithography.
The arrangement of nested reflectors as known from WO 99/27542 is used in an X-ray proximity lithography system in such a way that light of a light source is refocused so that a virtual light source is formed. The nested shells can have an ellipsoidal shape.
A nested reflector for high-energy photon sources is known from U.S. Pat. No. 6,064,072 which is used to shape the diverging X-rays into a parallel beam bundle.
WO 00/63922 shows a nested collector which is used to collimate a neutron beam.
A nested collector for X-rays is known from WO 01/08162 which is characterized by a surface roughness of the inner reflective surface, the individual mirror shells of less than 12 Årms. The collectors shown in WO 01/08162 also comprise systems with multiple reflections, especially also Wolter systems, and are characterized by a high resolution, as is also demanded in X-ray lithography for example.
In addition to resolution, high requirements are also needed regarding evenness, uniformity and telecentricity with respect to illumination lens systems for EUV lithography, such as in DE 199 03 907 or WO 99/57732.